1. Field of the Invention
The present invention relates to digital image and video processing. More specifically, the present invention relates to methods of converting frame rates for interlaced and progressive (i.e., non-interlaced) video streams.
2. Discussion of Related Art
Due to advancing semiconductor processing technology, integrated circuits (ICs) have greatly increased in functionality and complexity. With increasing processing and memory capabilities, many formerly analog tasks are being performed digitally. For example, images, audio and even full motion video can now be produced, distributed, and used in digital formats.
FIG. 1 is an illustrative diagram of a portion of interlaced digital video stream 100 most often used in television systems. Interlaced digital video stream 100 comprises a series of individual fields 100_1 to 100_N, of which the first ten fields are shown. Even fields contain even numbered rows while odd fields contain odd numbered rows. For example if a frame has 400 rows of 640 pixels, the even field would contains rows 2, 4, . . . 400 and the odd field would contains rows 1, 3, 5, . . . 399 of the frame. In general for an interlaced video stream each field is formed at a different time. For example, an interlaced video capture device (e.g. a video camera) captures and stores the odd scan lines of a scene at time T as field 100_1, then the video capture device stores the even scan lines of a scene at time T+1 as field 100_2. The process continues for each field.
Interlaced video systems were designed when bandwidth limitations precluded progressive (i.e., non-interlaced) video systems with adequate frame rates. Specifically, interlacing two 30 fps fields achieved an effective 60 frame per second frame rate because the phosphors used in television sets would remain “lit” while the second field is drawn. Progressive video streams use complete frames, including both the even and odd scan lines instead of fields. Because progressive scan provides better display quality, computer systems, which were developed much later than the original television systems, use progressive scan display systems. Furthermore, many modern televisions and television equipment are being developed to use progressive video streams. To maintain compatibility with existing interlaced video systems, modern progressive systems use deinterlacing techniques to convert interlaced video streams into progressive video streams.
FIG. 2(a) and 2(b) illustrate a typical method of generating a progressive video stream 200 from an interlaced video stream 100. Specifically each field 100_X of interlaced video stream 100 is converted to a frame 200_X of progressive video stream 200. The conversion of a field to a frame is accomplished by generating the missing scan lines in each frame by copying or interpolating from the scan lines in the field. For example, as illustrated in FIG. 2(b) field 100_1 having odd scan lines 100_1_1, 100_1_3, 100_1_5, . . . 100_1_N, is converted into a frame 200_1 by copying scan lines 100_1_X as odd scan lines 200_1_X, where X is an odd number and creating even scan lines 200_1_Y, where Y is an even number. Even scan lines 200_1_Y can be created by copying the preceding odd scan line 200_1_Y−1. This technique is commonly known as line repeat. Better results can be obtained using various interpolation schemes to generate the missing scan lines. For example, one interpolation scheme simply averages odd scan line 200_1_Y−1 with odd scan line 200_1_Y+1 to generate even scan line 200_1_Y. Other interpolation schemes may use weighted averages or other more complicated ways to combine data from the existing scan lines to generate the missing scan lines. Another normal mode deinterlacing technique known as 3D deinterlacing involves generating the missing scan lines by interpolating the missing pixels using data from adjacent fields. Conversion of fields into frames is not an integral part of the present invention. The principles of the present invention can easily be adapted to use any form of field to frame conversion.
However, many types of video streams are captured at different frame rates. For example, conventional motion pictures are captured and displayed using 24 frames per seconds. To display motion pictures on an NTSC (interlaced 60 fields/second) display, a frame rate conversion process that transforms four frames of a motion picture into ten fields is applied to the motion picture video stream. FIG. 3 illustrates this process. Specifically, FIG. 3 shows the first four frames M_01 to M_04 of a motion picture video stream MPVS being converted to 10 fields of an interlaced video stream 300. Field 300_1 and field 300_3 include the odd scan lines of frame M_01. Field 300_2 includes the even scan lines of frame M_01. Field 300_4 includes the even scan lines of frame M_02 and Field 300_5 includes the odd scan lines of frame M_02. Field 300_6 and field 300_8 include the even scan lines of frame M_03. Field 300_7 includes the odd scan lines of frame M_03. Field 300_9 includes the odd scan lines of frame M_04 and field 300_10 includes the even scan lines of frame M_04. For clarity, portions of interlaced video streams formed using frame rate conversion are referred said to be in “special mode.” Table 1 summarizes the relationship of the fields of interlaced video stream 300 and the frames of motion picture video stream MPVS.
TABLE 1FieldContent300_1ODD scan lines of MP_01300_2EVEN scan lines of MP_01300_3ODD scan lines of MP_01300_4EVEN scan lines of MP_02300_5ODD scan lines of MP_02300_6EVEN scan lines of MP_03300_7ODD scan lines of MP_03300_8EVEN scan lines of MP_03300_9ODD scan lines of MP_04300_10EVEN scan lines of MP_04
While displaying interlaced video stream 300 on interlaced video systems provide adequate picture quality. Conventional deinterlacing techniques as described above and illustrated in FIGS. 2(a) and 2(b) can be used by a progressive scan display system to view interlaced video stream 300. However, the picture quality of a de-interlaced video stream formed from interlaced video stream 300 is much lower than the picture quality of the original progressive video stream that was used to create interlaced video stream 300.
Hence, there is a need for a deinterlacing method or system that can determine whether a given interlaced video stream is in normal mode (e.g. like a normal television signal) or in a special mode (e.g. formed from a frame rate conversion process). The method or system must then deinterlace the given interlaced video stream appropriately.